There used to be some really good video of helicopter wake turbulence on the net - cannot find it this morning IIRC it was done by NASA.
Anyhow the standard stuff about WT still applies and if the wind was very light then it would persist for a 'long' time. With two similar aircraft it is quite likely that the wakes interacted to some extent.
In Aberdeen most people 'know' to avoid the stopping or rotation areas on the runway where choppers have been - even after the usual interval the air is pretty rough - especially on still days.